This invention relates to oscillator-type converters, and more particularly, to a free-running blocking oscillator-type converter which utilizes transformer feedback windings for controlling the conduction state of a switching transistor.
The reference Transistor-Gleichspannungswandler (Transistor dc Converters), by Helmut Schweitzer, 1969, page 34, teaches a free-running blocking oscillator-type converter which contains a switching transistor and a primary winding of a transformer connected in series across a pair of input terminals. The transformer is provided with a secondary winding which is connected by means of a rectifier and a smoothing filter to output terminals. A feedback winding is further provided in the transformer, and is connected at a first end to the base terminal of the switching transistor. The emitter terminal of the switching transistor is coupled directly to a reference potential, and the second end of the feedback winding is coupled to a terminal of a voltage divider at the input of the circuit. This feedback arrangement results in in an oscillation characteristic which is a function of the dynamic characteristics of the switching transistor. The reference does not teach the manner in which the output voltage of the blocking oscillator-type converter described therein would be controlled. The nonlinear characteristics of the switching transistor render the output voltage difficult to control by varying the bias at the base terminal.
Several possible arrangements for controlling switching power supplies are described in an article entitled "The Control of Switching Power Supplies" by R. Ranfft, in the lecture series "Switching Power Supplies", published by VALVO, in November, 1976. As disclosed therein, a series circuit consisting of an auxiliary transistor and a primary winding of a measuring transformer is disposed, for example, on the output side of a blocking oscillator-type converter. In this circuit, the auxiliary transistor is in a conductive state simultaneously with the switching transistor. The secondary winding of the measuring transformer is coupled to a series combination of a Zener diode and a voltage divider. The output voltage is compared with the Zener voltage, which serves as a reference potential. A controlled voltage, which is dc isolated from the output circuit, is available at a tap of the voltage divider. The article notes that dc isolation may be achieved by the use of optical couplers. However, the article does not describe the manner in which the switching transistor may be influenced by the control voltage.
It is, therefore, an object of this invention to provide a free-running blocking oscillator-type converter, the output voltage of which can be controlled in response to variations in the base-emitter of the switching transistor.